The present invention relates to a turbine bucket dovetail for assembly of buckets on steam turbine rotors and, more particularly, to a bucket dovetail where the dovetail shape introduces a design feature aimed at minimizing the concentrated stress (caused by the centrifugal force of buckets) in the wheel hook fillets while maintaining an overall size that is compatible with existing steam paths.
As a result of contaminated steam found in many existing turbines, the tangential entry dovetails on the latter stages of low pressure rotors operate in an environment that is conducive to stress corrosion cracking (SCC). SCC is accelerated by the stress levels that are present in the hook fillet region of typical dovetail configurations. Normally, these stresses are acceptable, but in contaminated steam, cracks can initiate and, left undetected, grow to a depth that will cause failure of wheel hooks. In extreme cases, all of the hooks will fail and buckets will fly loose from the rotor.
The tangential entry dovetail design necessitates an opening in the wheel periphery where the buckets are first inserted into the wheel radially and then slid tangentially into place. This wheel opening is defined as the “gate.” The gate is filled after all the buckets have been assembled on the wheel with a special bucket called a closure bucket. The closure bucket is secured in position with cross-keys. Cross-keys are required because of the absence of retaining hooks employed by the buckets in the row. The cross-key is passed through the “solid” portion of the closure piece and the adjacent bucket (auxiliary) above the wheel rim. At operating speed, the closure piece imposes an additional load on the auxiliary buckets by nature of the keying arrangement. The cross-key design transmits 50% of the closure piece centrifugal force to each auxiliary bucket. The centrifugal load from the closure/auxiliary buckets along with the stress concentration factors created by the abrupt change in dovetail geometry produces the peak wheel hook fillet stress at the gate.
Experience shows that wheel hooks crack and bucket hooks do not crack. This is because the NiCrMoV and similar low alloy steels used for low pressure rotors are much less resistant to SCC than are the 12 Cr steels used for buckets. NiCrMoV steels give the optimum combination of properties available for overall low pressure rotor design considerations. Thus, an effective means of avoiding SCC in the typical low pressure steam environment is to reduce the stresses in the wheel dovetail to acceptable levels. It is recognized by the turbine industry that if the peak stress in components operating in a corrosive environment is below the yield strength of the material, the resistance to SCC is greatly improved. Therefore, wheel dovetails can be designed to at least meet this criterion and strive to provide greater safety margins by reducing stresses even further.